阅读说明：本技术 一种超大尺寸铌酸锂单晶的生长方法 (Method for growing oversized lithium niobate single crystal ) 是由 孙德辉 王蒙 韩文斌 刘宏 于 2021-07-07 设计创作，主要内容包括：本发明公开了一种超大尺寸铌酸锂单晶的生长方法,包括以下步骤：(1)采用提拉法制备小尺寸晶体；在小尺寸晶体生长结束后提拉一段细颈；在细颈提拉结束后,进入扩肩、等径和收尾阶段,完成超大尺寸铌酸锂单晶的生长；(2)超大尺寸铌酸锂单晶生长结束后,冷却,将超大尺寸铌酸锂单晶与细颈分离,获得超大尺寸铌酸锂单晶。本发明解决了提拉法制备超大尺寸铌酸锂单晶在扩肩时,由中心热量对流不利而导致的凹陷这一现象,将显著满足新一代声学芯片大规模量产最亟需的战略大尺寸晶体材料的需求。(The invention discloses a method for growing an oversized lithium niobate single crystal, which comprises the following steps: (1) preparing small-size crystals by a Czochralski method; pulling a section of thin neck after the growth of the small-size crystal is finished; after the thin neck is pulled, the steps of shoulder expanding, equal diameter and ending are carried out to finish the growth of the lithium niobate single crystal with the oversized size; (2) and after the growth of the oversized lithium niobate single crystal is finished, cooling, and separating the oversized lithium niobate single crystal from the thin neck to obtain the oversized lithium niobate single crystal. The invention solves the problem of recess caused by unfavorable central heat convection when the ultra-large lithium niobate single crystal prepared by the pulling method is used for shoulder expansion, and can obviously meet the requirement of strategic large-size crystal materials which are most urgently needed by the large-scale mass production of new-generation acoustic chips.)

1. A method for growing an oversized lithium niobate single crystal is characterized by comprising the following steps:

(1) firstly, preparing small-size crystals by adopting a Czochralski method; pulling a section of thin neck after the growth of the small-size crystal is finished; then entering the stages of shoulder expanding, equal diameter and ending to finish the growth of the lithium niobate single crystal with the oversized size;

(2) and after the growth of the oversized lithium niobate single crystal is finished, cooling, and separating the oversized lithium niobate single crystal from the thin neck to obtain the oversized lithium niobate single crystal.

2. The growth method according to claim 1, wherein in step (1), the ratio of the small-size crystal diameter to the oversized lithium niobate single crystal diameter is (20-40): (200-300).

3. The growth method according to claim 2, wherein in the step (1), the small-sized crystals have a diameter of 20 to 30mm and a length of 8 to 9 mm.

4. The growing method according to claim 1, wherein in the step (1), the length of the narrow neck is 6-10mm, and the diameter of the narrow neck is 6-8 mm.

5. The growth method according to claim 1, wherein the Czochralski method for preparing the small-sized crystal in step (1) comprises: batching, mixing, sintering, melting, seeding, necking, shoulder expanding, constant diameter and ending.

6. The growing method according to claim 5, wherein the raw materials of the ingredients are lithium carbonate and niobium pentoxide, and the ingredients are mixed according to the molar ratio of the lithium to the niobium of 48.38/51.62.

7. The growth method according to claim 1, wherein in step (2), the oversized lithium niobate single crystal is separated from the narrow neck at a distance of 3 to 5mm from the narrow neck.

8. The growth method according to claim 1, wherein in the step (2), the diameter of the oversized lithium niobate single crystal is 200-300 mm.

Technical Field

The invention relates to the technical field of preparation of lithium niobate crystals, in particular to a method for growing an oversized lithium niobate single crystal.

Background

The fifth generation mobile communication technology (5G) system is accelerating the arrival of the fourth industrial revolution era, and is causing systematic change worldwide. The Chinese part 5G core technology is already in the first echelon of the global industry, but some key core devices still depend on import seriously. The radio frequency filter is the largest sub-industry in radio frequency front-end acoustic chips, and a common SAW uses a lithium niobate wafer as a substrate, so that the temperature drift is large, and the loss of a high-frequency band above 3.5GHz is large. In recent years, the acoustic application potential of lithium niobate single crystal thin film materials gradually gets wide international attention, TC-SAW with low temperature coefficient, IHP-SAW with high Q value and high-frequency Xbar can be realized, and the thin film has the advantages of integration and miniaturization, and can meet all performance requirements of a new generation of communication systems on radio frequency filters. The lithium niobate thin film is stripped from the lithium niobate wafer and depends on the bonding technology of the lithium niobate wafer and the silicon wafer; chip micromachining depends on a semiconductor micromachining production line, and the current main production line of the semiconductor is 8 inches; therefore, the 8-inch lithium niobate crystal conforming to the semiconductor production line is the most critical strategic crystal material for large-scale mass production of new-generation acoustic chips.

Specifically, the lithium niobate crystal is grown mainly by Czochralski method, in which a sintered lithium niobate raw material is placed in a platinum crucible and heated to melt, seeding is performed by a seed crystal rotation pulling method, and then the lithium niobate single crystal is subjected to necking, shouldering, isodiametric sizing, and ending processes. When the pulling method is used for preparing the lithium niobate single crystal, the situation that the shoulders are wide and the waist is thin often exists, which is very unfavorable for the large-scale growth of the lithium niobate single crystal, and the problems that a large amount of waste is caused from the crystal to the crystal bar exist, and the like exist. However, for the growth of lithium niobate single crystals with oversized sizes (8 inches, 10 inches and 12 inches) closely related to the process of the semiconductor industry, under the influence of the large size, the situation can be firstly changed in the shoulder expanding process, namely, the shoulder expanding process is unfavorable due to central heat convection, so that the center of the crystal has a very serious concave phenomenon, the shape of the crystal is similar to that of a pot cover, and the phenomenon can cause the crystal to stop growing. This situation is extremely disadvantageous for the growth of oversized lithium niobate crystals, thereby limiting further applications.

Disclosure of Invention

Aiming at the prior art, the invention aims to provide a method for growing an oversized lithium niobate single crystal. The invention solves the problem of recess caused by unfavorable central heat convection when the ultra-large lithium niobate single crystal prepared by the pulling method is used for shoulder expansion, and can obviously meet the requirement of strategic large-size crystal materials which are most urgently needed by the large-scale mass production of new-generation acoustic chips.

In order to achieve the purpose, the invention adopts the following technical scheme:

a method for growing an oversized lithium niobate single crystal comprises the following steps:

(1) firstly, preparing small-size crystals by adopting a Czochralski method; pulling a section of thin neck after the growth of the small-size crystal is finished; then entering the stages of shoulder expanding, equal diameter and ending to finish the growth of the lithium niobate single crystal with the oversized size;

(2) and after the growth of the oversized lithium niobate single crystal is finished, cooling, and separating the oversized lithium niobate single crystal from the thin neck to obtain the oversized lithium niobate single crystal.

Preferably, in the step (1), the ratio of the diameter of the small-sized crystal to the diameter of the oversized lithium niobate single crystal is (20-40): (200-300).

More preferably, in step (1), the small-sized crystals have a diameter of 20 to 30mm and a length of 8 to 9 mm.

Preferably, in the step (1), the length of the thin neck is 6-10mm, and the diameter of the thin neck is 6-8 mm.

Preferably, in step (1), the Czochralski method for preparing small-sized crystals comprises: batching, mixing, sintering, melting, seeding, necking, shoulder expanding, constant diameter and ending.

More preferably, the raw materials of the material are lithium carbonate and niobium pentoxide, and the material is prepared according to the molar ratio of the lithium to the niobium of 48.38/51.62.

Preferably, in the step (2), the oversized lithium niobate single crystal is separated from the narrow neck at a distance of 3-5mm from the narrow neck.

Preferably, in the step (2), the diameter of the oversized lithium niobate single crystal is 200-300 mm.

The invention has the beneficial effects that:

the invention increases the heat dissipation of the central axial direction by the method of continuously growing the lithium niobate crystal with the oversize size at the final stage of the ending of the small crystal. The central heat dissipation of the oversized lithium niobate single crystal which does not contain small crystals and directly grows is mainly transferred by the seed crystals, and the seed crystals are too small compared with the oversized lithium niobate crystals, and the diameter ratio of the seed crystals is more than 40 times, so that the axial heat dissipation capacity is not outstanding, and the grown crystals are sunken. The method for pulling the small crystal firstly is adopted, so that the problem that the central liquid level is sunken when the large-size crystal is directly grown is solved, and meanwhile, the utilization rate of the small crystal can be increased by cutting off the large-size crystal after the growth of the oversized crystal is finished, so that the growth period is further shortened; or the small crystals are returned to the furnace for treatment, thereby further fully improving the utilization rate of the raw materials. Meanwhile, for the growth habit of wide shoulders and thin waist of the crystal, the invention accelerates the quality change of the crystal in the shoulder expanding process by accelerating the heat dissipation of the central axial direction, so that the crystal enters the equal diameter stage more quickly, the time for shoulder putting to the equal diameter can be shortened on the basis of improving the equipment parameters, and the defect of shoulder width is further solved in an optimized manner, so that the raw materials can be better utilized and the industrial preparation can be realized.

Drawings

FIG. 1 is a schematic view of a small-sized crystal structure grown first according to the present invention;

FIG. 2 is a schematic diagram of the structure of an 8-sized crystal obtained after growing a small-sized crystal according to the present invention;

FIG. 3 is a schematic view of a small-sized crystal structure of the present invention which is recycled after cutting.

FIG. 4 is a physical photograph of an 8-inch lithium niobate crystal prepared in example 1 of the present invention.

FIG. 5 is a growth-interrupted "pot-lid" type primary crystal prepared in comparative example 1 of the present invention; in the figure, A and B are respectively the front and back surfaces of the pot-lid type primary crystal.

Detailed Description

It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the disclosure. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.

As introduced in the background section, oversized (8 inch, 10 inch, 12 inch) lithium niobate single crystals are the most critical strategic crystalline material needed for mass production of new generation acoustic chips. In the prior art, lithium niobate crystals are mainly grown by a Czochralski (Czochralski) pulling method. When the pulling method is used for preparing the lithium niobate single crystal with the oversized dimension, the situation that the shoulders are wide and the waist is thin often exists, the quality of the single crystal is influenced, and the method is very unfavorable for the large-scale growth of the lithium niobate single crystal; and latent heat released during crystal growth must be conducted away from the vicinity of an interface, so that the crystal can keep stable growth, for the growth of the lithium niobate crystal with the super-large size, the situation can be firstly changed in the shoulder expanding process under the influence of the large size, the central heat is unfavorable for convection during shoulder expanding, so that the center of the crystal has a very serious concave phenomenon, the shape of the crystal is similar to that of a pot cover, and the phenomenon can cause the crystal to stop growing.

Therefore, it is very difficult to grow the lithium niobate single crystal with the oversized dimension by the existing pulling method.

Based on the above, the invention aims to provide a method for growing a lithium niobate crystal with an oversized size, which aims to accurately solve the problem of central depression in the shoulder expanding process of the crystal with the oversized size so as to meet the growth requirement of the crystal with the oversized size. The main inventive concept of the invention is as follows: before the oversized lithium niobate crystal grows, pulling a small crystal, so that the axial heat dissipation of the oversized lithium niobate crystal can be increased through the small crystal, and the problem that the growth is stopped due to the central depression of the oversized lithium niobate crystal is effectively solved through the acceleration of the central axial heat dissipation.

In one embodiment of the present invention, a method for growing oversized lithium niobate crystals is provided, comprising the steps of:

(1) the raw materials are mixed according to the selection of the mixture ratio, and the molar ratio of lithium to niobium is 48.38/51.62;

(2) briquetting the prepared materials, and then putting the materials into a muffle furnace for high-temperature sintering;

(3) putting the sintered raw materials into a platinum crucible container, compacting, and putting into a pulling furnace for melting;

(4) slowly lowering the seed crystal to a position 20mm away from the liquid level, and baking the seed crystal;

(5) immersing the seed crystal into the liquid level, melting part of the seed crystal at a proper seeding temperature, and then pulling;

(6) after seeding, pulling up 1mm, and necking, in which the width is smaller than that of the seed crystal;

(7) after the necking and 1mm pulling are finished, a small crystal growth shoulder expanding stage is carried out, and when the shoulder expanding degree is larger than the width of the seed crystal, program automatic control is carried out to finish the equal diameter and ending stage of the small crystal (figure 1);

(8) and at the end of the ending stage of the small crystal, pulling a section of narrow neck, then entering an oversize crystal growth shoulder expanding stage under the condition of the set crystal form, and then finishing the equal diameter and ending stage (figure 2).

After the growth of the oversized crystal is finished, the oversized crystal is cut from the position of 3-5mm of the thin neck, and the oversized crystal and the small crystal are obtained by separation (figure 3), and the separated small crystal can be directly used or the small crystal is put back to the furnace for treatment, so that the utilization rate of raw materials is more fully improved.

In order to make the technical solutions of the present application more clearly understood by those skilled in the art, the technical solutions of the present application will be described in detail below with reference to specific embodiments.

The test materials used in the examples and comparative examples of the present invention are conventional in the art and are commercially available.

Example 1:

a preparation method of an oversized lithium niobate crystal comprises the following steps:

s1, firstly drying 5N-grade lithium carbonate and 5N niobium pentoxide serving as lithium niobate growth raw materials to remove moisture in air, and selecting a lithium-niobium molar ratio of 48.38/51.62 for proportioning;

s2, mixing the prepared lithium niobate raw material in a mixer for 24 hours;

s3, briquetting the mixed material, and sintering the briquetted material in a muffle furnace at 1200 ℃, wherein the sintering time is 14 hours;

s4, after the raw material is sintered, putting the sintered raw material into a platinum crucible, after melting and stabilizing, lowering the seed crystal with the size of 6 x 50mm to a position 20mm away from the upper part of the liquid level, and keeping the seed crystal rotating at the rotating speed of 7 revolutions per minute;

s41, after the melt temperature is stable, enabling the seed crystal to enter the liquid level for 1mm, stabilizing for 10min after entering the liquid level for 1mm, starting to pull and neck, wherein the pulling speed during necking is 2mm/h, the heating speed is 2 ℃/h, the neck width is 4mm, and the pulling height is 10 mm;

s43, after necking, slowly cooling at a cooling rate of 10 ℃/h, entering a shoulder expanding stage, reducing the pulling speed to 1mm/h, and expanding the diameter of the crystal to 20 mm; then slowly raising the temperature, wherein the temperature raising rate is 5 ℃/h, then entering an equal-diameter stage of small crystals, keeping the temperature constant, and pulling at an equal-diameter pulling speed of 1mm/h, after the equal-diameter height reaches 9mm, raising the temperature raising rate by 10 ℃/h, entering a finishing stage, after the diameter of the crystals is reduced to 8mm, keeping the temperature stable and unchanged, pulling a section of thin neck at a pulling speed of 2mm/h, wherein the length of the thin neck is 8mm, and the diameter of the thin neck is 6 mm.

S5, then entering a growth stage of the 8-inch lithium niobate crystal, and completing the growth of the lithium niobate crystal with the oversized size through shoulder expanding, equal diameter and ending stages;

s51, shoulder expanding stage related parameters: the cooling rate is 10 ℃/h, the pulling speed is 1mm/h, the crystal diameter is enlarged to 205mm, and then the heating rate is 8 ℃/h, and the diameter is equal;

s52, equal-diameter stage related parameters: the cooling rate is 5 ℃/h, the pulling speed is 0.9mm/h, and the equal-diameter pulling height is 70 mm.

S53, relevant parameters of the ending stage: the heating rate is 8 ℃/h, the pulling speed is up to 1mm/h, and after the temperature is kept for 1 hour, the crystal is separated from the melt at the pulling speed of 200 mm/h.

S6, finally, the temperature is reduced to the room temperature at the speed of 50 ℃/h.

S7, cutting the grown 8-inch lithium niobate crystal from the position 3-5mm away from the narrow neck, and separating to obtain the 8-inch lithium niobate crystal (FIG. 4).

Comparative example 1:

s1, firstly drying 5N-grade lithium carbonate and 5N niobium pentoxide serving as lithium niobate growth raw materials to remove moisture in air, and selecting a lithium-niobium molar ratio of 48.38/51.62 for proportioning;

s2, mixing the prepared lithium niobate raw material in a mixer for 24 hours;

s3, briquetting the mixed material, and sintering the briquetted material in a muffle furnace at 1200 ℃, wherein the sintering time is 14 hours;

s4, after the raw materials are sintered, putting the raw materials into a platinum crucible, and after melting and stabilizing, descending the seed crystal to a position 20mm away from the position above the liquid level;

s41, after the melt temperature is stable, enabling the seed crystal to enter the liquid level for 1mm, stabilizing for 10min after entering the liquid level for 1mm, starting to pull and neck, wherein the pulling speed during necking is 2mm/h, the heating speed is 2 ℃/h, the neck width is 4mm, and the pulling height is 10 mm;

s42, after necking, slowly cooling at a cooling rate of 10 ℃/h, entering a shoulder expanding stage, reducing the pulling speed to 1mm/h, and expanding the diameter of the crystal to 205 mm; then slowly raising the temperature, wherein the temperature raising rate is 5 ℃/h, the process is shifted to an equal-diameter stage, the temperature is constant, the equal-diameter drawing speed is 1mm/h, a concave interface is easy to appear on a growth interface at the moment, and the growth is interrupted under severe conditions. The crystal is in the shape of a "pot lid" (fig. 5A, B).

Comparative example 2:

the diameter of the small-sized crystal prepared in step S43 of example 1 was adjusted to 15mm, the isometric height was adjusted to 6mm, and the rest of the operation was unchanged.

Comparative example 3:

the length of the neck pulled in step S43 of example 1 was adjusted to 12mm, the diameter of the neck was adjusted to 10mm, and the rest of the operation was not changed.

Comparative example 4:

the length of the neck pulled in step S43 of example 1 was adjusted to 4mm, the diameter of the neck was adjusted to 4mm, and the rest of the operation was not changed.

The 8-inch lithium niobate crystals were repeatedly produced 20 times by the methods of example 1, comparative example 1 to comparative example 4, and the number of times that "pot-lid" shaped crystals appeared during the production was recorded, with the results shown in table 1:

table 1:

group of	Number of appearance of "pot lid" shaped crystals
		Example 1	0
Comparative example 1	18
		Comparative example 2	4
Comparative example 3	1
		Comparative example 4	2

The above results show that: the method can obviously improve the success rate of preparing the lithium niobate crystal with the oversized size, is favorable for realizing the large-scale production of the lithium niobate crystal with the oversized size, and avoids the problem that the growth is stopped because the center of the growth of the lithium niobate crystal with the oversized size is sunken.

The above description is only a preferred embodiment of the present application and is not intended to limit the present application, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, improvement and the like made within the spirit and principle of the present application shall be included in the protection scope of the present application.